Does Mars have rights? What about Europa, Ganymede, and Titan—the moons of Jupiter and Saturn that may be home to rudimentary extraterrestrial life? The 1967 Outer Space Treaty requires spacefaring nations to conduct exploration of the moon and other celestial bodies “so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter.” The goal of the treaty is to prevent both back contamination (the introduction of extraterrestrial life to Earth) and forward contamination (the introduction of Earth life to extraterrestrial environments).

The reason for avoiding back contamination is pretty clear. We want to prevent an Andromeda Strain scenario in which an unleashed alien life form harms life on Earth. Returning Apollo astronauts and their hauls of moon rocks were quarantined for a couple of weeks, just to make sure that no lunar microbes escaped to wreak havoc. Years of testing found no indication of life hidden in the moon rocks.

The main reason to guard against forward contamination is to prevent equipment designed to detect extraterrestrial life from getting confused. Consequently, NASA regularly sterilizes gear destined to land on other celestial bodies. So far no mission has detected life anywhere else in our solar system.